Re: [PATCH 2/6] x86: bug: prevent gcc distortions

[Nadav Amit]

Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> 
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
>> On Thu, May 17, 2018 at 09:13:58AM -0700, Nadav Amit wrote:
>>> +asm(".macro __BUG_FLAGS ins:req file:req line:req flags:req size:req\n"
>>> +    "1:\t \\ins\n\t"
>>> +    ".pushsection __bug_table,\"aw\"\n"
>>> +    "2:\t "__BUG_REL(1b)		"\t# bug_entry::bug_addr\n\t"
>>> +    __BUG_REL(\\file)			"\t# bug_entry::file\n\t"
>>> +    ".word \\line"			"\t# bug_entry::line\n\t"
>>> +    ".word \\flags"			"\t# bug_entry::flags\n\t"
>>> +    ".org 2b+\\size\n\t"
>>> +    ".popsection\n\t"
>>> +    ".endm");
>>> +
>>> +#define _BUG_FLAGS(ins, flags)                                          \
>>> do {									\
>>> +	asm volatile("__BUG_FLAGS \"" ins "\" %c0 %c1 %c2 %c3"		\
>>> +		     : : "i" (__FILE__), "i" (__LINE__),                \
>>> +			 "i" (flags),                                   \
>>> 			 "i" (sizeof(struct bug_entry)));		\
>>> } while (0)
>> 
>> This is an awesome hack, but is there really nothing we can do to make
>> it more readable? Esp, that global asm doing the macro definition is a
>> pain to read.
>> 
>> Also, can we pretty please used named operands in 'new' code?
> 
> Yes, that's my main worry too about all these inlining changes:
> the very, very marked reduction in the readability of assembly code.
> 
> It's bad to an extent that I'm questioning the wisdom of pandering to a compiler 
> limitation to begin with?
> 
> How about asking GCC for an attribute where we can specify the inlined size of an 
> asm() function? Even if we'll just approximate it due to some vagaries of actual 
> code generation related to how arguments interact with GCC, an explicit byte value 
> will do a heck of a better job of it than some sort of implied, vague 'number of 
> newlines' heuristics ...

If it were to become a GCC feature, I think it is best to be a builtin that
says: consider the enclosed expression as âfreeâ. The problem of poor
inlining decisions is not specific to inline asm. As I mentioned in the RFC,
when there are two code paths for constants and variables based on
__builtin_constant_p(), you can get the âcostâ of the constant path for
variables.

It is not hard to add such a feature to GCC, but I donât know how easy it is
to get new features into the compiler.